/*<
 * Copyright 2002-2017 the original author or authors.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package org.springframework.aop;

import org.springframework.lang.Nullable;

/**
 * A {@code TargetSource} is used to obtain the current "target" of
 * an AOP invocation, which will be invoked via reflection if no around
 * advice chooses to end the interceptor chain itself.
 *
 * <p>If a {@code TargetSource} is "static", it will always return
 * the same target, allowing optimizations in the AOP framework. Dynamic
 * target sources can support pooling, hot swapping, etc.
 *
 * <p>Application developers don't usually need to work with
 * {@code TargetSources} directly: this is an AOP framework interface.
 *
 * @author Rod Johnson
 * @author Juergen Hoeller
 */
public interface TargetSource extends TargetClassAware {

    /**
     * Return the type of targets returned by this {@link TargetSource}.
     * <p>Can return {@code null}, although certain usages of a {@code TargetSource}
     * might just work with a predetermined target class.
     *
     * @return the type of targets returned by this {@link TargetSource}
     */
    @Override
    @Nullable
    Class<?> getTargetClass();

    /**
     * Will all calls to {@link #getTarget()} return the same object?
     * <p>In that case, there will be no need to invoke {@link #releaseTarget(Object)},
     * and the AOP framework can cache the return value of {@link #getTarget()}.
     *
     * @return {@code true} if the target is immutable
     * @see #getTarget
     */
    boolean isStatic();

    /**
     * Return a target instance. Invoked immediately before the
     * AOP framework calls the "target" of an AOP method invocation.
     *
     * @return the target object which contains the joinpoint,
     * or {@code null} if there is no actual target instance
     * @throws Exception if the target object can't be resolved
     */
    @Nullable
    Object getTarget() throws Exception;

    /**
     * Release the given target object obtained from the
     * {@link #getTarget()} method, if any.
     *
     * @param target object obtained from a call to {@link #getTarget()}
     * @throws Exception if the object can't be released
     */
    void releaseTarget(Object target) throws Exception;

}
